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J-Pole Problem

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W4XKE, Apr 21, 2012.

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  1. K7JEM

    K7JEM Ham Member QRZ Page

    I think someone needs to test a J pole against a QW antenna and make some accurate measurments. I'm sure the J pole is better than some people seem to think, and worse than others think. It would be interesting to see the results, with and without a balun.

    I have built J poles and talked to people using them, and they seem to operate as a decent antenna. Nothing special, but not worse than a QW in the cases I have seen. There are some construction issues that make them easier to build and deploy than a QW, in most cases. Rather than just talk, someone needs to make some actual comparisons.

    Joe
     
  2. W0UZR

    W0UZR Ham Member QRZ Page

    The J part of your antenna is the counterpoise part of your antenna, and your connections are so far up that there isn't enough of it. that's why you have a good match when you are grabbing the antenna.

    Take a solid wire, like from house wire, strip a decent chunk and fold it in half. Now solder the ends at the point where your connections are on the J stub where your shield is soldered, and loop the wire around evenly making sure it's the same distance out all the way around. Try that and let me know what you get.
     
    Last edited: Apr 23, 2012
  3. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    This falls into the don't believe everything you read on the internet. A half wave vertical antenna does not require a counterpoise. You can look that up in the ARRL handbook but a properly designed J-Pole antenna should not require any ground at all in order to provide low angle radiation. The matching section, if it is working properly, is not a radiating element. Because the J-Pole uses a balanced feed it is necessary to use a current or choke balun as close to the matching section as practical.
     
  4. W0BTU

    W0BTU Ham Member QRZ Page

    I respectfully but strongly disagree. The only half-wave vertical antennas that do not require a counterpoise or ground radials are:

    1. A center-fed dipole on its side
    2. A properly designed coaxial dipole, the kind with the hollow bottom element.

    EVERY end-fed antenna requires a counterpoise, unless you don't mind the issues resulting from RF flowing on the outside of your coax shield. Period. See http://www.w0btu.com/Optimum_number_of_ground_radials_vs_radial_length.html#noground

    Few things are more well-established than that fact. The high Z at the end of a /2 vertical does not negate the need for an RF ground.
     
  5. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    While I don't want to coin a phrase, the truth is, popularity doesn't relate in any way to good design. The fact is, a J-pole is a lousy antenna. Its attributes are that it is easy to build, and the presence of common mode is given. Trying to choke off the common mode with an ugly balun doesn't work well, especially at VHF. And why should you need one? A 1/4 wave doesn't, and in most cases a 1/4 wave will out perform a J-pole.

    I also agree with Mike. After all, RF must return to its source. For a J-pole, or a 1/2 wave dipole (a true disambiguation), that's back down the outside of the coax as common mode.
     
  6. W0BTU

    W0BTU Ham Member QRZ Page

    You said it much better than I did, Alan. :)
     
  7. K7JEM

    K7JEM Ham Member QRZ Page

    This is the part I would like to see more info on. I have read all kinds of stuff about J poles being lousy antennas, and other people who seem to think they are the greatest. I think the truth is somewhere in between. I have never seen a really scientific test done, in real world, comparing a center fed dipole, a J pole, and a QW antenna. Test the J pole with and without a balun.

    I have talked to many people over the decades that used them to just discount their performance. Even if it has the same gain as a QW, there may be many applications where a J pole may be a better choice.

    Joe
     
  8. W0BTU

    W0BTU Ham Member QRZ Page

    This is kind of a complex issue. One of the things about a J-pole is that there are more variables than a λ/4 ground plane, due to the common-mode currents on the outside of the coax shield.

    This means that not only is a significant portion of the radiation coming from the coax shield, but that same shield is also more susceptible to picking up unwanted noise that can mask the signal we're trying to hear.

    That's right: on a J-pole, the coax is part of the antenna. That's what many people overlook. The radiation does not just come from the part that looks like an antenna.

    If the mast is conductive, the proximity of the coax to the mast as well as the length of the mast can greatly affect how the J-pole works. A good λ/4 (or other length) ground plane at the top of any mast has no such issues.

    If you're only trying to work local repeaters, a J-pole is probably OK. But if you're interested in working farther out, and especially weak signal simplex, that's where other omnidirectional antennas in a 'drag race' usually pull away from the J-pole.

    And since a ground plane is probably simpler to make than a J-pole, why not just make a ground plane in the first place? :)
     
  9. K7JEM

    K7JEM Ham Member QRZ Page

    This may be all correct. But if the J pole works, in whatever configuration it is, then it may be adequate for its intended purpose, and shouldn't be arbitrarily discounted for every ham, or every use.

    Joe
     
  10. W0BTU

    W0BTU Ham Member QRZ Page

    I guess that's not a bad way of looking at things. I suppose if you already have a J-pole and are having fun, that's the main thing. :)

    It's just that the J-pole is notoriously unpredictable, as the OP (and probably thousands of other hams) found out. And since that's the case, why bother in the first place?

    Here's something else to think about. There is only ONE commercial manufacturer of non-amateur J-pole antennas, and he started fairly recently. IIRC, he's a ham.

    I think one of the reasons people are attracted to the J-pole is because it's not as "familiar" as other omnis. Hey, it's different, so it must be better. :rolleyes:
     
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